1. Field of the Invention
The invention relates generally to quality control inspection of ferrous metal parts and more particularly to wet particle systems in which fine ferrous particles in oil are bathed onto the part for flaw detection.
(2) Description of the Prior Art
In the prior art, the basic technique of detecting faults or cracks (flaws) in a manufactured ferrous metal part by magnetizing the part and bathing it in a slurry of fine particles of iron in a vehicle of oil. The faults or flaws tend to be visually observable in that they form a characteristic interruption of the otherwise uniform pattern of the iron particles (filings) clinging magnetically to the magnetized part. That technique has sometimes been referred to as wet magnetic particle inspection.
As a general rule, the detection of discontinuities in ferrous materials is accomplished by generating a suitable magnetic field normal to the effected direction of the discontinuity.
U.S. Pat. No. 2,242,366 disclosed a relatively early system for dealing with the drawbacks of still earlier systems which could not detect longitudinal cracks parallel to the axis of an elongated part being inspected when the magnetization is axially applied. Similarly, transverse cracks are not detectable when the magnetization is of the circular type, such as produced by axial current flow in the part itself. In the aforementioned U.S. Pat. No. 2,212,366 (Muller) a system is disclosed in which at least one of the fields is generated by an AC current component producing an "oscillating" net field. Although workable, this system has been found to be wanting because of AC field cancellation effects under certain conditions. Moreover, any system applying a high intensity magnetic field in the longitudinal direction of the part is subject to eddy current effects which set up magnetic anomalies and can also cause heating of the part.
For locating discontinuities (cracks) at various angles on the surface of a part or within its interior, it is necessary to generate both circular and longitudinal magnetic fields. The circular field is generated by passing a current axially through the part if it is generally of cylindrical or annular cross-section, or in case of a hollow part, such as a gun barrel, a longitudinal wire within the hollow volume can also be used to generate the circular field. A longitudinal magnetic field has usually been generated in prior art devices by a loop-type electromagnet having continuous, windings in planes generally normal to the axis of the part.
Another example of a prior art system for the purpose is disclosed in U.S. Pat. No. 3,034,021 (Callihan). In that system the currents for both the axial current through the part (for circular field generation) and through a single peripheral (loop-type) electromagnet coil are supplied in parallel. Thus, individual control of those currents is not possible.
In U.S. Pat. No. 2,242,366 (Muller) aforementioned, the two magnetizing currents are likewise not individually controllable.
Various prior art arrangements using saturable reactors with selenium rectifier stacks are known in variations of the basic prior art systems. More recently, silicon diodes replaced selenium stacks for high-efficiency, smaller size and lower cost in the power supply configurations.
U.S. Pat. No. 4,443,759 (Isaacson et al) discloses the use of SCR elements in a still more recent version of an apparatus for the purpose. Thus, the concept of controlling the relatively large magnetizing currents by "firing phasing" in SCR elements is known. However, again the concept of separate control of the two components of magnetizing current escaped recognition. Still further, uniformity of magnetization in the longitudinal direction is considerably less effective in single magnetizing coil configurations. U.S. Pat. No. 4,443,759 falls in that category. The same is true of the Callihan and Muller systems discussed hereabove.
AC magnetizing current arrangements are known, but generally perform less well than contemporaneous DC systems.
The manner in which the system according to the invention deals with prior art disadvantages and shortcomings will be evident as this specification proceeds.